The problem is, I think, both devices are doing rekey at the same time.
In that case, “get event” should show many “Bad SPI” outputs.

The workaround is:
1. adjusting when to start rekey (soft lifetime buffer) on the Juniper device.
2. set ike initiator-set-commit/ set ike responder-set-commit (Both are needed. Let the peer device know which SA should be used).